In recent years, reducing the amount of silver to be coated in a silver halide photographic material has been an important subject from the standpoint of saving resources.
As a general rule, a silver halide photographic material containing a specific silver halide emulsion in a given amount per unit area shows some definite value of optical density or covering power of developed silver.
The optical density generally increases with an increase in the amount of silver coated per unit area of a silver halide photographic material, and decreases with a decrease therein. Accordingly, if the amount of silver to be coated per unit area of silver halide photographic material is intended to be reduced the technique developed must also prevent a decrease in optical density when the amount of silver coated per unit area of the silver halide photographic material is reduced. More specifically, the technique must raise the covering power of the developed silver.
These are a number of generally known methods which involve adding polysaccharides to silver halide emulsion layers, protective layers or interlayers of silver halide photographic materials with the intention of raising covering powers of the silver halide photographic materials. For example, such methods are disclosed in U.S. Pat. Nos. 3,762,924; 3,063,838; 3,137,575; 3,069,267; 3,085,010; 3,087,818; 3,152,906; 3,185,569 and 3,441,412: British Pat. Nos. 1,063,841; 1,105,478 and 1,126,798: German Pat. Nos. 1,211,939; 1,286,399; 1,293,579 and 2,525,591: and so on. Other known methods involve adding hydrophilic polymers to photosensitive silver halide emulsion layers, protective layers or interlayers of silver halide photographic materials with the same intention as described above. Examples of these methods are disclosed in U.S. Pat. Nos. 3,178,296; 3,165,412; 3,271,158; 3,312,554; 3,514,289; 3,393,083; 3,316,097; 3,597,215; 3,591,387; 3,058,826; 3,043,698; 3,408,199 and 4,030,929: and so on.
However, the addition of a substance as described above is not desirable because it deteriorates physical properties of the silver halide photographic material. For example, surfaces of the silver halide photographic material are liable to be scratched during development-processing, the hydrophilic polymer added or gelatin used as a binder eludes into processing solutions, or the transparency of the photographic material is lowered by receiving processings. However, such methods can raise the covering power of developed silver of the silver halide photographic material.
Techniques for surmounting the above-described with the polysaccharides, in which in addition to the polysaccharides, hydrophilic polymers are further added, are disclosed in British Pat. No. 976,221: U.S. Pat. Nos. 3,203,804 and 3,514,289: and so on. However, all these techniques are not enough to completely prevent the above-described deterioration of physical properties of silver halide photographic materials.